Mn 1s Research Articles

Experimental and computational X-ray emission spectroscopy as a direct probe of protonation states in oxo-bridged Mn(IV) dimers relevant to redox-active metalloproteins.

The protonation state of oxo bridges in nature is of profound importance for a variety of enzymes, including the Mn4CaO5 cluster of photosystem II and the Mn2O2 cluster in Mn catalase. A set of dinuclear bis-μ-oxo-bridged Mn(IV) complexes in different protonation states was studied by Kβ emission spectroscopy to form the foundation for unraveling the protonation states in the native complex. The valence-to-core regions (valence-to-core XES) of the spectra show significant changes in intensity and peak position upon protonation. DFT calculations were performed to simulate the valence-to-core XES spectra and to assign the spectral features to specific transitions. The Kβ(2,5) peaks arise primarily from the ligand 2p to Mn 1s transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite Kβ" peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated μ-oxo bridges dominate this spectral region. The energies of the Kβ" features differ by ~3 eV and thus are well resolved in the experimental spectra. Additionally, our work explores the chemical resolution limits of the method, namely, whether a mixed (μ-O)(μ-OH2) motif can be distinguished from a symmetric (μ-OH)2 one. The results reported here highlight the sensitivity of Kβ valence-to-core XES to single protonation state changes of bridging ligands, and form the basis for further studies of oxo-bridged polymetallic complexes and metalloenzyme active sites. In a complementary paper, the results from X-ray absorption spectroscopy of the same Mn(IV) dimer series are discussed.

The protonation states of oxo-bridged Mn(IV) dimers resolved by experimental and computational Mn K pre-edge X-ray absorption spectroscopy.

In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-μ-oxo-bridge protonation in a Mn dimer is represented by the system [Mn(IV)2(salpn)2(μ-OHn)2](n+). Although the three species [Mn(IV)2(salpn)2(μ-O)2], [Mn(IV)2(salpn)2(μ-O)(μ-OH)](+) and [Mn(IV)2(salpn)2(μ-OH)2](2+) differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretically calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high spin (HS) and broken symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn 1s core electrons into the unoccupied orbitals of local e(g) character (d(z)(2) and d(xy) based in the chosen coordinate system). The lowest energy experimental feature is dominated by excitations of 1s-α electrons, and the second observed feature is primarily attributed to 1s-β electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn-O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in photosystem II.

Modeling Near-Edge Fine Structure X-ray Spectra of the Manganese Catalytic Site for Water Oxidation in Photosystem II

The Mn 1s near-edge absorption fine structure (NEXAFS) has been computed by means of transition-state gradient-corrected density functional theory (DFT) on four Mn(4)Ca clusters modeling the successive S(0) to S(3) steps of the oxygen-evolving complex (OEC) in photosystem II (PSII). The model clusters were obtained from a previous theoretical study where they were determined by energy minimization. They are composed of Mn(III) and Mn(IV) atoms, progressing from Mn(III)(3)Mn(IV) for S(0) to Mn(III)(2)Mn(IV)(2) for S(1) to Mn(III)Mn(IV)(3) for S(2) to Mn(IV)(4) for S(3), implying an Mn-centered oxidation during each step of the photosynthetic oxygen evolution. The DFT simulations of the Mn 1s absorption edge reproduce the experimentally measured curves quite well. By the half-height method, the theoretical IPEs are shifted by 0.93 eV for the S(0) → S(1) transition, by 1.43 eV for the S(1) → S(2) transition, and by 0.63 eV for the S(2) → S(3) transition. The inflection point energy (IPE) shifts depend strongly on the method used to determine them, and the most interesting result is that the present clusters reproduce the shift in the S(2) → S(3) transition obtained by both the half-height and second-derivative methods, thus giving strong support to the previously suggested structures and assignments.

X-ray absorption near-edge structure of GaN with high Mn concentration grown onSiC

By means of x-ray absorption near-edge structure (XANES) severalGa1−xMnxN (0.03<x<0.09) layers have been analyzed. The Mn-doped GaN samples consisted of different epilayersgrown by molecular beam epitaxy on [0001] SiC substrates. The low mismatch betweenGaN and SiC allows for a good quality and homogeneity of the material. Themeasurements were performed in fluorescence mode around both the Ga and MnK edges. All samples studied present a similar Mn ionization state, very close to2+, and tetrahedral coordination. In order to interpret the near-edge structure, we haveperformed ab initio calculations using the full potential linear augmented plane wavemethod as implemented in the Wien2k code. The calculations show the appearance of a Mnbonding band localized in the gap region, and the corresponding anti-bonding state , which seem to be responsible for the double structure which appears at the pre-edgeabsorption region. The shoulders and main absorption peak of the XANES spectra areattributed to transitions from the Mn(1s) band to the conduction bands, whichare partially dipole allowed because of the Mn(4p) contribution to these bands.

Origin of the chemical shift in X‐ray absorption near‐edge spectroscopy at the Mn K‐edge in manganese oxide compounds

AbstractThe absorption edge in Mn K‐edge X‐ray absorption spectra of manganese oxide compounds shows a shift of several electronvolts in going from MnO through LaMnO3 to CaMnO3. On the other hand, in X‐ray photoelectron spectra much smaller shifts are observed. To identify the mechanisms that cause the observed chemical shifts, 1s ionization as well as 1s → “4p” transition energies have been determined by electronic structure calculations on embedded Mn ions and embedded MnO6 clusters. Systematic variation of the cluster geometry and the cluster embedding showed that the chemical shifts are predominantly determined by two effects: the changes in the Mn 3d occupation and the changes in the Madelung potential. The large chemical shift in the 1s → 4p transition energies between different materials occurs because the two effects do not compensate each other. The chemical shifts obtained for the embedded MnO6 clusters agree reasonably with the experimental shifts. The small sensitivity to the material observed for the Mn 1s ionization energies is explained by the near cancellation of the effects of the Madelung potential and the 3d occupation of the Mn ion. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Anisotropic valence→core x-ray fluorescence from a [Rh(en)3][Mn(N)(CN)5]⋅H2O single crystal: Experimental results and density functional calculations

High resolution x-ray fluorescence spectra have been recorded for emission in different directions from a single crystal of the compound [Rh(en)3][Mn(N)(CN)5]⋅H2O. The spectra are interpreted by comparison with density functional theory (DFT) electronic structure calculations. The Kβ″ line, which is strongly polarized along the Mn–N axis, can be viewed as an N(2s)→Mn(1s) transition, and the angular dependence is understood within the dipole approximation. The so-called Kβ2,5 region has numerous contributions but is dominated by Mn(4p) and C(2s)→Mn(1s) transitions. Transition energy splittings are found in agreement with those of calculated occupied molecular orbitals to within 1 eV. Computed relative transition probabilities reproduce experimentally observed trends.

An evaluation of structural models for the photosynthetic water-oxidizing complex derived from spectroscopic and X-ray diffraction signatures.

Four of the five intermediate oxidation states (S-states) in the catalytic cycle of water oxidation used by O2-evolving photoautotrophs have been previously characterized by EPR and/or ENDOR spectroscopy, with the first reports for the S0, S1, and S3 states available in just the last three years. The first electron density map of the Mn cluster derived from X-ray diffraction measurements of single crystals of photosystem II at 3.8-4.2 A resolution has also appeared this year. This wealth of new information has provided significant insight into the structure of the inorganic core (Mn4OxCa1Cl1-2), the Mn oxidation states, and the location and function of the essential Ca2+ cofactor within the water-oxidizing complex (WOC). We summarize these advances and provide a unified interpretation of debated structural proposals and Mn oxidation states, based on an integrated analysis of the published data, particularly from Mn X-ray absorption spectroscopy (XAS) and EPR/ENDOR data. Only three magnetic spin-exchange models for the inter-manganese interactions are possible from consideration of the EPR data for the S0, S1, S2 and S(-N) (NO-reduced) states. These models fall into one of three types denoted butterfly, funnel, or tetrahedron. A revised set of eight allowed chemical structures for the Mn4Ox core can be deduced that are shown to be consistent with both EPR and XAS. The popular "dimer-of-dimers" structural model is not compatible with the possible structural candidates. EPR data have identified two inter-manganese couplings that are sensitive to the S-state, suggesting two possible bridging sites for substrate water molecules. Spin densities derived from 55Mn hyperfine data together with Mn K-edge energies from Ca-depleted samples provide an internally consistent assignment for the Mn oxidation states of Mn4(3III,IV) for the S2 state. EPR and XAS data also provide a consistent picture, locating Ca2+ as an integral part of the inorganic core, probably via shared bridging ligands with Mn (aqua/hydroxo/carboxylato/chloro). XAS data reveal that the Ca2+ cofactor increases the Mn(1s-->4p) transition energy by 0.6-1 eV with minimal structural perturbation versus the Ca-depleted WOC. Thus, calcium binding appears to increase the Mn-ligand covalency by increasing electron transfer from shared ligands to Mn, suggesting a direct role for Ca2+ in substrate water oxidation. Consideration of both the XAS and the EPR data, together with reactivity studies on two model complexes that evolve O2, suggest two favored structure types as feasible models for the reactive S4 state that is precursor to the O2 evolution step. These are a calcium-capped "cuboidal" core and a calcium-capped "funnel" core.

Chemical dependence of interatomic X-ray transition energies and intensities – a study of Mn Kβ″ and Kβ 2, 5 spectra

A study of ligand (N, O, F) 2s to metal (Mn) 1s `interatomic' or `crossover' X-ray transitions is reported. We show that the energy of the Kβ″ feature is related to the ligand 2s binding energy and can be used to identify the type of ligand. For oxygen-ligated Mn compounds, the strength of the Kβ″ transition decreases exponentially with increasing Mn–O distance. This can be used to predict distances to ∼0.1 Å. The Kβ 2, 5 energy shows shifts of ∼1 eV per unit oxidation state. Kβ″ and Kβ 2, 5 transitions are a promising tool for structural characterization of transition-metal complexes.